1. Field of the Invention
The invention relates in general to a semiconductor device and a method of manufacturing the same, and more particularly to a semiconductor device having a non-volatile memory and a method of manufacturing the same.
2. Description of the Related Art
A non-volatile memory (NVM) is a memory that can store information even when not powered. Examples of non-volatile memory include a read-only memory (ROM) and a flash memory according to whether the stored data can be modified or not. Data is written into the ROM during initial manufacturing process. Once the ROM is completed, the ROM can only be read, and the stored data cannot be changed or removed. On the contrary, the flash memory is programmed, erased or read by providing voltage. In other words, data stored in the flash memory can be removed or rewritten anytime whenever a user wants to. Flash memories are applied to various consumer electronics due to the advantages of compact size, light weight and low electricity consumption.
Lately, owing to the requirements of lightweight and multi-function consumer electronic products, the electronic products need to be smaller, lighter and more efficient. However, the efficiency of the electronic product utilizing a flash memory as the storage media is affected by the programming/erasing rate of the memory.
Generally, in the silicon-oxide-nitride-oxide-silicon (SONOS) stack structure of the flash memory, a first silicon dioxide (SiO2) layer, a silicon nitride (SiN) layer, a second silicon dioxide (SiO2) layer and a polysilicon layer are formed on a semiconductor substrate orderly. Data stored in the memory depends on the electrons in the silicon nitride layer. By applying voltage among a control gate, a drain, a source and the silicon substrate, a strong electric field is generated for removing or accumulating electrons in the silicon nitride layer. As a result, the data stored in the memory can be changed. In the erase operation, electrons stored in the silicon nitride layer are removed. In the programming operation, electrons are accumulated in the silicon nitride layer. In the SONOS stack structure, the thickness of the first silicon dioxide layer has to be less than 3 nanometers (nm) for maintaining the Fowler-Nordheim erase rate. The thin first dioxide layer (such as 2 nm to 3 nm) facilitates the erase operation, such as channel hole injection. However, the electrons or holes are still injected from the channel even when the electric field is low. The thin first silicon dioxide layer also increases the possibility of losing electrons or holes stored in the silicon nitride. As a result, data retention property of the memory is degraded.
In the above-described SONOS stack structure, the data erase rate affects the operation efficiency of the memory significantly. Therefore, it is very important to increase the data erase rate of the memory effectively for improving the operation efficiency of the flash memory and to maintain good data retention ability of the memory at the same time.